A/Ab is a prefix. Its meaning from, away, or away from. By the way there are articles in Latin but less than in English. In English there is a,an and the.
As a prefix, "ab-" is used to mean "away from" or "off", from the Latin word "ab" meaning "away from". For example, the word abnormal is used to denote something which is outside or away from the realm of normalness.